This usually happens when driving up steep inclines. The engine will rev fairly high and then there will be a shudder and loud clunk as it jams itself into the next gear.

There are several problems with this car of mine. Steering clunks and grinds, rotors are warped after 12,000 miles, fan belt squeals like mad, even with a/c off, and now the transmission. The trans slips horribly bad from reverse to drive or vice versa. If the car is cold or hot, either way, it is bad. I will put it in drive after backing out of a parking space, and it takes a couple of seconds to find drive. And when it does, CLUNK into gear.

I have a list of fixes for the dealer.